Plate tectonics

Earth's outer shell is not a single solid piece. It is broken into several large pieces called tectonic plates.

These plates make up the lithosphere, which is the rigid outer layer of our planet. This layer includes the crust and the very top part of the mantle. There are seven or eight major plates on Earth. There are also many smaller pieces called platelets. These plates are always moving very slowly. They move between zero and 10 cm every year. This speed is about the same as how fast your fingernails grow.

These plates move because of what happens deep inside the Earth. The plates float on a layer called the asthenosphere. This layer is hotter and flows more easily than the plates. Heat from the mantle creates convection currents. These are slow, creeping motions in the solid mantle. As the plates move, they create different types of boundaries. At divergent boundaries, plates slide apart from each other. This creates new ground through seafloor spreading. At convergent boundaries, plates move toward each other. One plate might slide under another in a process called subduction.

Scientists have studied these movements for a long time. The idea of moving continents was discussed in the early 20th century. Alfred Wegener was a scientist who worked on these ideas.

He shared his thoughts during the first decades of that century. Later, in the mid- to late 1960s, scientists found proof of seafloor spreading. This discovery helped everyone accept the theory of plate tectonics. Now, geoscientists use this model to understand how our planet works. It explains why the Earth's surface is always changing.

There are many important facts about how these plates act. At some places, like the Mid-Atlantic Ridge, plates move very slowly. At other places, like the Nazca plate, they move about 16 cm a year.

The plates can be made of oceanic crust or continental crust. Oceanic crust is denser and sits lower in the ocean. Continental crust is less dense and sits higher above sea level. When plates grind past each other at transform boundaries, they can cause earthquakes. The San Andreas Fault in California is a famous example of this.

Plate tectonics connects many things we see in nature. Most of the world's active volcanoes happen at plate boundaries. The Pacific plate has a famous area called the Ring of Fire.

This area is very active with volcanic and earthquake activity. When plates crash together, they can push up huge mountain ranges. The Himalayas and the Alps were formed this way. Even other worlds might have similar movements. Jupiter's moon Europa shows signs of moving ice plates. This shows that the idea of moving plates is a very special part of how planets work.